Police responded with stun grenades and bursts of tear gas to disperse the anarchists, who were split into two groups - one of them mixed among a peaceful protest of about 10,000 people holding banners and the other inside Syntagma Square in front of parliament.

The bill, introduced as part of requirements debt-ridden Greece country must meet under its third international bailout, is set to dramatically increase social security and pension contributions and raise taxes for most people. Greek workers say the increases will decimate their incomes, already hurt by six years of crippling austerity, and have staged a series of strikes.

The larger protest on Sunday was called for by Greece's biggest unions and most of those in attendance were Communist party sympathisers.

About 45 minutes before they started throwing projectiles, anarchists approached and beat up a known farmer activist, shouting that he was a member of the far-right Golden Dawn party. Other protesters dragged the farmer away.

A few minutes later, another group of anarchists set upon another person, with riot police using stun grenades to stop the beating. The anarchists then regrouped before attacking riot police outside a trio of luxury hotels in Syntagma Square.

The police use of tear gas initially cleared the space outside the parliament, but most of the peaceful protesters have returned, determined to stay until politicians vote on a pension and taxation reform bill later on Sunday.

There were also clashes in Thessaloniki, when anarchists broke away from a protest march to hurl fire bombs at police guarding the local offices of the ruling Syriza party. Police used tear gas and chased the rioters through the streets in central Thessaloniki.

Several professional associations, including engineers, journalists and doctors, have warned politicians that are also members of these associations, that they will face disciplinary action and possible expulsion if they vote for the bill. Greece's prime minister Alexis Tsipras himself is a member of the engineers' association.